The customer demand in LDPE (low density polyethylene) plastics has rapidly been increased over the time, which has resulted in continuous upscaling of LDPE plant design. The train capacity of the first generation high pressure LDPE trains ranged from 3 to 10 KTA, which was scaled up to 60 to 80 KTA during the 1970's (G. Luft, Chem.-Ing.-Tech., Hochdruck-Polyaethylen, Vol. 51 (1979) Nr. 10, pages 960-969). The train capacity was further raised to 200 KTA in the 1990's, and nowadays the LDPE trains with capacities exceeding 350 KTA are being designed and operated. While in the past a LDPE plant consisted of multiple LDPE reactor trains, for example, four trains of 50 KTA each. Nowadays a LDPE plant can consist of a single 400 KTA train. Larger trains of 600 KTA or more are foreseeable. A LDPE plant with multiple trains is more easily adapted to changing ethylene, power supply and customer demand, while the daily polymer output will be less impacted by reliability issues at a train level. The single large train operation results in lower investment and operation costs; however large swings in daily polymer output, due to reliability issues, places heavier demands on the reliability of ethylene and the power supply.
It is well-known that conventional low density polyethylene (LDPE) is produced with high pressure (for example, 160 MPa to 400 MPa) technology, either in autoclave and/or tube reactors. The initiator systems, known as free-radical agents, are typically injected at multiple points along the reactor, thus creating multiple reaction zones in autoclave and/or tubular reactor systems. The polymerization usually takes place by the feeding of free-radical initiator system at temperatures in the range from 130° C. to 360° C.
It is expected that during normal operation, the train is operated at a preferred set of process conditions, in order to maximize the production rate. However, in some situations, the output of the train has to be reduced (or stopped) due to internal or external problems. Internal train problems could include (temporarily) failures in extrusion, pelletization, compression or pellet transport systems, while external problems could include reduced ethylene supply by problems in the ethylene production, storage or transport systems. Another reason to reduce train output could be a temporarily and/or seasonal lack of demand for LDPE product. Lack of demand could force a plant to go down, from time to time, and to produce and to store product, in advance of a prolonged train stop, in order to secure product supply. This would necessitate investment in storage capability for extended product stock.
Shutting down a high pressure train is not preferred, since it will result in a considerable loss of production during a stop and start-up. Also, additional effort is needed to start up a plant, more off-grade product is produced, and the risk of having process instabilities, such as reactor fouling or even run away reactions (ethylene decomposition), increases.
Publication No. WO 2010/081685 describes a method for reducing the output of a high pressure tubular reactor for the homo- or copolymerization of ethylene, in the presence of free-radical polymerization initiator systems, with at least two initiator injection points, where the feeding of free-radical polymerization initiator system to the first injection point is disconnected, or reduced, to a level, such that the temperature of the reaction mixture in the first reaction zone does not rise more than 20° C. However, a big drawback of the proposed methodology is that polymer properties, for example long chain branching, short chain branching, the molecular weight distribution, as well as density, are not controlled, and will widely vary, which result in production of off-grade product. An off-grade product has at least one significantly property feature outside product specification.
There remains a need for a new polymerization process that can be used to reduce the polymer output at constant product quality, when there is a need to reduce the output of a LDPE train, due to reduced product demand, a reduced supply of a feed stock, or some other restriction(s). There is also an important need for a new process that can produce high pressure ethylene-based polymers, which have maintained polymer properties during the period when output is reduced. These needs have been met by the following invention.